Recently, display apparatus using organic ELs and the like as light emitting elements have been actively developed for portable information terminals or for television receivers. A self-emissive display apparatus including, for each pixel, a light emitting element such as an organic EL has good visibility and superior motion-image display characteristic.
An example of a conventional display apparatus using the organic ELs as light emitting elements is known, by way of example, from Japanese Patent Laying-Open No. 11-212493.
FIG. 37 is a circuit diagram representing a configuration of the conventional display apparatus described in the afore-mentioned laid-open application, in which four signal lines (Sm, 1˜Sm, 4) and four scanning lines (Dn, 1˜Dn, 4) are connected to a light emitting element (m, n) through thin film transistors TFT1˜4. Further, constant current sources (Im, 1˜Im, 4) are connected to the signal lines (Sm, 1˜Sm, 4), and by setting current ratio thereof to 1:2:4:8, the current to the light emitting element is controlled to have 16 different values, to attain emission luminance in 16 different tones.
A so called active type display apparatus has been widely known, which uses a thin film transistor (TFT) formed on a glass substrate as a switching element of a pixel. An active type display apparatus using light emitting elements of which emission luminance varies with current such as organic ELs is particularly advantageous over a passive type one that does not use a switching element for a pixel, in that higher luminance can be attained with smaller driving current to a light emitting element, because, based on a re-written signal, a current can be kept flowing through the light emitting element until the next time of rewriting.
Of thin film transistors, a low temperature poly-silicon TFT (low temperature p-Si TFT) that can be manufactured through a low temperature process has higher electron mobility than an amorphous silicon TFT. Therefore, it is possible to form a driving circuit thereof integrally with a pixel matrix circuit on a glass substrate, and hence, it has come to be widely used for liquid crystal display apparatus and the like.
It is noted, however, that the low temperature p-Si TFT, which is generally formed by laser annealing, has Vth (threshold voltage) and μ (mobility) varied more widely than single-crystal silicon, because it is difficult to uniformly control laser irradiation intensity over the glass substrate.
In the conventional display apparatus, a plurality of constant current sources are connected to every signal line of each column. Therefore, when the constant current sources are formed in the display panel integrally with the pixel matrix on the glass substrate using TFTs, variation of TFT characteristics causes variation in the output currents from the constant current sources of respective columns and hence variation in signal line driving currents. This results in unevenness in emission luminance.
Further, it is necessary to arrange a plurality of signal lines for every column, and therefore, wiring layout becomes difficult in a display apparatus of high resolution having a narrow pixel pitch.
In a general configuration, luminance tone of each pixel is designated by digital image data. Therefore, when the number of bits of the image data increases together with the increase in colors to be displayed, voltage variation of an image data line transmitting the image data may possibly affects generation of the signal line driving current on a signal line supplying a current to a light emitting element.